Pivot joints arranged for facilitating that structural elements such as an antenna, a mast or like may be pivoted between an upraised position and a lowered position is generally exposed to significant forces in essentially all directions. The antenna or like may itself be quite heavy and in upraised position also the height of the antenna, especially when being exposed to e.g. wind, imply great stress to the joint where the pivotally antenna is fastened. For applications where the antenna or like is fastened by a pivot joint to a vehicle the pivot joint will also be exposed to other forces e.g. during transport in harsh terrain. In order for the pivot joint to work properly at such conditions, not wear down to quickly and not be dislocated the material thickness of the components associated with the pivot joint often have to be larger than what is desired and the requirements regarding manufacturing tolerances are often very restrictive. Small tolerances and excessive material thickness are undesirable from a cost and weight perspective.
Conventional pivot joints for pivotally antennas or like generally consist of a pivot axle which is rotationally arranged at respective end in a pivot axle fastener. Both pivot axle fasteners contributes to absorbing forces in radial direction exerted on the pivot axle but only one of the pivot axle fasteners absorb the forces if being applied in a direction essentially parallel with the pivot axle. Thus, if such a pivot joint is exposed to shock forces, for example during transport when driving on a bumpy road, in an axial direction there is an imminent risk that the pivot axle will be separated from the pivot axle fastener if not the pivot axle fastener is sufficiently rigid.
However, it is not sufficient that the pivot axle fasteners are sufficiently rigid. Also the structural element on which the pivot axle fasteners are arranged needs to be able to absorb the forces exerted by the pivot axle fasteners. If being arranged on a vehicle, such structural element may e.g. be a load platform of a truck or the roof of a terrain vehicle. Thus also the load platform, the roof etc. needs to be designed or reinforced in order to prevent the pivot axle from been dislocated from axial forces.
When in upraised position, and during operation when the antenna is rotating, another aspect that needs to be considered is the mechanical resonance. Mechanical resonance is the tendency of a mechanical structure to respond at greater amplitude when the frequency of its oscillations matches the natural frequency of the structure. A rotating antenna exposed to incident wind will be exposed to a pulsating force due to that the surface of the antenna that is exposed to the incident wind will vary as the antenna rotates. Such pulsating force may cause the antenna structure to start to oscillate. If such oscillations coincide with the natural frequency of the structure the oscillations will escalate which ultimately may break the antenna structure. For a radar antenna the performance of the antenna will also be significantly impaired by such oscillations. The oscillation caused is dependent on the rotating speed of the antenna.
A rigid fastening, without any glitches, raises the natural frequency wherein the natural frequency may be raised to be higher than the frequencies which the antenna structure may be exposed to due to rotating in windy conditions. A rigid antenna structure and rising of the natural frequency improves the stability and durability of the antenna. Also, the more stable the antenna is, the better, and more accurate, will e.g. a radar image obtained by such an antenna be.
In order for a pivot joint to work properly it is also important that the inherent pivot resistance, counteracting the pivot movement of the joint, is configured accurately. For applications where a pivot joint is used for lowering and raising antennas or like the inherent pivot resistance is important for dampening the rotational movement since the leverage effect is significant, especially when lowering the antenna. However, as the pivot joint is being worn the inherent pivot resistance will gradually decrease, wherein the dampening effect will be lost.
Thus, there is a need for further improvements.